U.S. Pat. No. 4,025,186 to Hunt, Jr. et al is representative of a number of publications and commercial apparatus in which indicia on an endless web is used to control timing of a reproduction apparatus. In that apparatus an electrophotographic web has a series of perforations (sometimes herein called "perfs") along one edge. The perforations are sensed at a position along the path of the web and the resulting indications of the presence of a perforation are sent to a logic and control means which controls the timing of various portions of the apparatus. The logic and control means may include a clock which creates an underlying set of clock pulses which are used to control the timing of the machine. The clock is updated periodically by the indications from the sensing means.
Although timing of all stations of an electrophotographic apparatus can be controlled in response to indicia sensing, two stations, image formation and image transfer are especially critical. Use of a series of perforations has a preciseness that is dependent upon the preciseness of location of the perforations. For ordinary reproduction using either optical or electronic exposure, the accuracy of perforation location expected from the photographic industry is adequate. However, some applications require more preciseness than this ordinary perforation formation provides. For example, if successive images are to be superposed on a single surface at the transfer station to form a multicolor image, precise registration of those images governs the quality of the multicolor image. Similarly, if successive images are to be used as color separation masters in xeroprinting, lithoprinting, or the like, and if the edges of the masters are to be used for registration of such images, location of the images on the masters is more critical than ordinary perf formation provides.
A publication, IBM Technical Disclosure Bulletin, Vol. 28, No. 7, Dec. 1985, page 2942 describes a laser printer having a single mark on an endless photoconductor belt which is sensed to synchronize the timing of printer operations. Because of inaccuracies in the size of the belt, the distance of travel between sensing the single mark as it repeatedly passes the sensor is measured by an encoder. The encoder is then used for timing the operation of the machine. The distance measured between consecutive sensings of the mark is used to create a correction signal to adjust the timings when the mark occurs in the middle of a cycle.
This publication suggests that a single mark per belt is preferred to a mark for each frame because of problems associated with manufacturing and maintaining positional tolerance between multiple marks on a flexible belt. It also requires an encoder for continual day-to-day operation.
Other apparatus have also been suggested in the literature in which a single mark per frame is used to trigger timing with a plurality of sensors one for each operational function to be timed, see for example, U.S. Pat. No. 3,606,532, Shelfo. In these structures timing is dependent upon accurate relative location of the critical sensors.